1. Field of the Invention
This invention relates generally to wavelength stabilization of integrated optical components or elements integrated on semiconductor chips or integrated in monolithic photonic integrated circuits (PICs) and more particularly to the manner of monitoring and controlling of wavelength tuning elements in or associated such chips or PICs.
Reference in this disclosure to wavelength stabilization is generally to stabilizing the lasing wavelengths of a plurality of laser sources, such as DFB or DBR lasers, on a monolithic TxPICs having different operational wavelengths approximating a standardized wavelength grid, such as the ITU grid. Further, this application relates to optimization of the laser source wavelength grid with the optical multiplexer or combiner wavelength grid where the array of laser sources and multiplexer are integrated on the same PIC. Also, further, this application relates to creating the required output power comb versus the wavelengths of the modulated sources. As used herein, modulated sources may be comprised of directly modulated (DM) lasers or externally modulated lasers, such as SMLs, e.g., EMLs.
2. Description of the Related Art
If used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
1R—Re-amplification of the information signal.
2R—Optical signal regeneration that includes signal reshaping as well as signal regeneration or re-amplification.
3R—Optical signal regeneration that includes signal retiming as well as signal reshaping as well as regeneration or re-amplification.
4R—Any electronic reconditioning to correct for transmission impairments other than 3R processing, such as, but not limited to, FEC encoding, decoding and re-encoding.
A/D—Add/Drop.
APD—Avalanche Photodiode.
AWG—Arrayed Waveguide Grating.
BER—Bit Error Rate.
CD—Chromatic Dispersion.
CDWM—Cascaded Dielectric wavelength Multiplexer (Demultiplexer).
CoC—Chip on Carrier.
DBR—Distributed Bragg Reflector laser.
EDFAs—Erbium Doped Fiber Amplifiers.
DAWN—Digitally Amplified Wavelength Network.
DCF—Dispersion Compensating Fiber.
DEMUX—Demultiplexer.
DFB—Distributed Feedback laser.
DLM—Digital Line Modulator.
DM—Direct Modulation.
DON—Digital Optical Network as defined and used in this application.
EA—Electro-Absorption.
EAM—Electro-Absorption Modulator.
EDFA—Erbium Doped Fiber Amplifier.
EML—Electro-absorption Modulator/Laser.
EO—Electrical to Optical signal conversion (from the electrical domain into the optical domain).
FEC—Forward Error Correction.
GVD—Group Velocity Dispersion comprising CD and/or PMD.
ITU—International Telecommunication Union.
MMI—Multimode Interference combiner.
Modulated Sources—EMLs or SMLs, combinations of lasers and external modulators or DM lasers.
MPD—Monitoring Photodiode.
MZM—Mach-Zehnder Modulator.
MUX—Multiplexer.
NE—Network Element.
NF—Noise Figure: The ratio of input OSNR to output OSNR.
OADM—Optical Add Drop Multiplexer.
OE—Optical to Electrical signal conversion (from the optical domain into the electrical domain).
OEO—Optical to Electrical to Optical signal conversion (from the optical domain into the electrical domain with electrical signal regeneration and then converted back into optical domain) and also sometimes referred to as SONET regenerators.
OEO-REGEN—OEO signal REGEN using opto-electronic regeneration.
OO—Optical-Optical for signal re-amplification due to attenuation. EDFAs do this in current WDM systems.
OOO—Optical to Optical to Optical signal conversion (from the optical domain and remaining in the optical domain with optical signal regeneration and then forwarded in optical domain).
OOO-REGEN—OOO signal REGEN using all-optical regeneration.
OSNR—Optical Signal to Noise Ratio.
PIC—Photonic Integrated Circuit.
PIN—p-i-n semiconductor photodiode.
PMD—Polarization Mode Dispersion.
REGEN—digital optical signal regeneration, also referred to as re-mapping, is signal restoration, accomplished electronically or optically or a combination of both, which is required due to both optical signal degradation or distortion primarily occurring during optical signal propagation caused by the nature and quality of the signal itself or due to optical impairments incurred on the transport medium.
Rx—Receiver, here in reference to optical channel receivers.
RxPIC—Receiver Photonic Integrated Circuit.
SDH—Synchronous Digital Hierarchy.
SDM- Space Division Multiplexing.
Signal regeneration (regenerating)—Also, rejuvenation. This may entail 1R, 2R, 3R or 4R and in a broader sense signal A/D multiplexing, switching, routing, grooming, wavelength conversion as discussed, for example, in the book entitled, “Optical Networks” by Rajiv Ramaswami and Kumar N. Sivarajan, Second Edition, Morgan Kaufmann Publishers, 2002.
SMF—Single Mode Fiber.
SML—Semiconductor Modulator/Laser.
SOA—Semiconductor Optical Amplifier.
SONET—Synchronous Optical Network.
SSC—Spot Size Convert, sometimes referred to as a mode adapter.
TDM—Time Division Multiplexing.
TEC—Thermal Electric Cooler.
TRxPIC—Monolithic Transceiver Photonic Integrated Circuit.
Tx—Transmitter, here in reference to optical channel transmitters.
TXPIC—Transmitter Photonic Integrated Circuit.
VOA—Variable Optical Attenuator.
WDM—Wavelength Division Multiplexing. As used herein, WDM includes Dense Wavelength Division Multiplexing (DWDM).
It is known in the art to provide a photonic integrated circuit (PIC) chip comprising a plurality of aligned semiconductor lasers lasing at different wavelengths forming a wavelength grid of outputs which are optically coupled on the chip through passive waveguides to an optical combiner or multiplexer, where the combined output is generally amplified. Examples of such a PIC is disclosed in the paper of M. Bouda et al. entitled, “Compact High-Power Wavelength Selectable lasers for WDM Applications”, Conference on Optical Fiber Communication, Technical Digest series, Vol. 1, pp. 178-180, Mar. 7-10, 2000, Baltimore Md., showing a ¼-shift DFB laser array optically coupled to a multi-mode interference (MMI) optical combiner with a semiconductor optical amplifier (SOA) to amplify the combined output. Another example is the article of Bardia Pezeshki et al. entitled, “12 nm Tunable WDM Source Using an Integrated Laser Array”, Electronic Letters, Vol. 36(9), pp. 788-789, Apr. 27, 2000 also showing a ¼-shift DFB laser array optically coupled to a multi-mode interference (MMI) optical combiner with an optical amplifier to amplify the combined or multiplexed output. A further paper is to M. G. Young et al. entitled, “A 16×1 Wavelength Division Multiplexer with Integrated Distributed Bragg Reflector lasers and Electroabsorption Modulators”, IEEE Photonics Technology Letters, Vol. 5(8), pp. 908-910, August, 1993 which disclosed an integrated PIC having modulated sources comprising DBR lasers and electro-absorption modulators (EAMs) coupled to a combiner with its output provided to an AR coated PIC facet via an SOA on-chip amplifier. Other examples are disclosed in U.S. Pat. No. 5,394,489 (modulated combiner output via an electro-absorption modulator); U.S. Pat. No. 5,612,968 (redundant DFB lasers); U.S. Pat. No. 5,805,755 (multiple combiner outputs); and U.S. Pat. No. 5,870,512 (modulated combiner output via a Mach-Zehnder modulator).
Also, known in the art is the integration in a single monolithic optical chip, i.e., a photonic integrated circuit (PIC), a plurality of semiconductor optical amplifiers (SOAs) with their optical outputs coupled via a plurality of passive waveguides to an AWG optical multiplexer to form a multiple wavelength laser source having multiple established laser cavities including these coupled optical components. See, for example, the paper of Charles H. Joyner et al., entitled, “Low-Threshold Nine-Channel Waveguide Grating Router-Based Continuous Wave Transmitter”, Journal of Lightwave Technology, Vol. 17(4), pp. 647-651, April, 1999. To be noted is that there is an absence in the art, at least to the present knowledge of the inventors herein, of the teaching of an integrated laser modulated source array, such as in the form of modulated sources and wavelength selective optical multiplexer, e.g., such as an arrayed waveguide grating (AWG) or Echelle grating. In this disclosure, a wavelength selective multiplexer or combiner is defined as one that has less than 1/N insertion loss wherein N is the number of modulated sources being multiplexed. The principal reason is that it is difficult to fabricate, on a repeated basis, an array of DFB lasers with a wavelength grid that simultaneously matches the wavelength grid of the a wavelength selective combiner (e.g., an AWG). The prior art is replete with control systems to control the temperature of laser diodes to control their temperatures, examples of which are disclosed in U.S. Pat. Nos. 5,949,562; 6,104,516; and 6,233,262 as well as in the article of D. Alfano entitled, “System-On-Chip Technology Adds Options for Laser Driver Control”, WDM Solutions, pp. 43-48, November, 2001, as well as the control of DFB laser arrays as seen in published U.S. patent application US2001/0019562A1, published Sep. 6, 2001. Also, there are control systems to control the temperature of the wavelength grid of an AWG as set forth in U.S. Pat. No. 5,617,234.
Also, known in the art is a monolithic chip comprising the integration of plurality of distributed feedback (DBR) semiconductor lasers operating at different wavelengths with their outputs provided to an optical multiplexer in the form of an array waveguide grating (AWG) as disclosed in the article of S Ménézo et al. entitled, “10-Wavelength 200-GHz Channel Spacing Emitter Integrating DBR Lasers with a PHASAR on InP for WDM Applications”, IEEE Photonics Technology Letters, Vol. 11(7), pp. 785-787, July, 1999. DBR laser sources are employed in the chip rather than DFB laser sources because they can be tuned to fit the wavelength comb of the AWG. However, these types of laser sources are more difficult to manufacture in an array and in monolithic form compared to DFB laser sources. But again, the integration of a DFB laser array with an AWG optical multiplexer with matching of their respective wavelength grids is difficult to achieve. Furthermore, none of these reference demonstrates the combination of modulated sources, such as, a modulated laser source (either directly modulated or externally modulated) with any type of source laser (DFB or DBR) in combination with a frequency selective multiplexer or combiner. Such sources are advantages as they provide the possibility of extremely high transmission capacities with the lowest optical loss and hence are part of the current invention.
Recently, U.S. Pat. No. 6,301,031 discloses an apparatus for wavelength channel tracking and alignment in an optical communication system. The disclosure of the '031 patent is directed to an optical combiner and feedback detection device preferably formed on the same substrate and a plurality of transmitter lasers having outputs coupled to the optical combiner. Part of the multiplexed signals from the optical combiner are tapped and provided to the input of the detection system which monitors the channel wavelengths to determine if the any one of the operating laser signal wavelengths is offset from its desired wavelength in a predetermined or standardized wavelength grid. The system also monitors a reference wavelength, λ0, relative to the standardized wavelength grid to determine if the reference wavelength is offset from its desired wavelength in a standardized wavelength grid. Thus, two different sets of wavelengths are to be aligned to a standardized wavelength grid. First and second feedback loops, provided from detectors at the outputs of the detection system, respectively provide for alignment of the passband of the optical combiner, via the detected reference wavelength, λ0, to a standardized wavelength grid and alignment of the respective wavelengths of the transmitter lasers to a desired wavelength on a standardized wavelength grid. Feedback signals affect an operating parameter of the laser sources and optical combiner, most notably their operating temperature where their operating wavelengths and passband, respectively, change due to changes in refractive index of their as-grown materials with ambient temperature variations. Patent '031 is, further, directed to monitor the output power of the multiplexed signals and adjustments are undertaken to the operating temperature and/or current of the transmitter lasers to optimize their power output. While the patent suggests that it is within the ability of those skilled in the art to provide such a monitoring system to change the operating temperatures of these optical components, other than detecting power, such as null crossing, tone detection, and the use of a wavelength selective device for the detection device, such as, wavelength routers, optical filtering device, fiber gratings or Fabry-Perot etalons, there is no disclosure or direction given as to how such a wavelength adjustment and feedback system may be implemented, particularly in the case where, importantly, the multiple transmitter lasers and the optical coupled optical combiner are both provided on the same substrate as a monolithic photonic integrated circuit (PIC).
Lastly, patent '031 indicates that the crux of the invention is not related to how the optical components are secured, whether discrete devices or combined on a single substrate, as the attributes of the invention would apply to both such cases. However, there is no disclosure how the invention is to be accomplished in the case of full integration of these optical components on a single PIC chip, in particular, what problems are encountered in such an integration and still achieve a wavelength control system with the dual function of monitoring and adjusting the individual wavelengths of the transmitter wavelengths to a standardized grid as well as the passband of the optical multiplexer to the same standardized grid.
It is an object of this invention to provide an optical transmitter for wavelength stabilization of integrated optical components or elements integrated on semiconductor chips or integrated in monolithic photonic integrated circuits (PICs).